有人可以告诉我为什么该程序打印-56吗？

Question

This code prints -56 and I'm not sure why

#include<stdio.h>

main()
{
    printf("%d\n", ((int)(char)200));
}

Answer 1

Because char implicitly is signed char on your platform.

Do

printf("%d\n", ((int)(unsigned char)200));

to get

200

Answer 2

• The type char is inconsistent towards the rest of the C language, since char can be either signed or unsigned. The C standard states it as implementation-defined behavior, meaning that the compiler can implement it in either way. Your particular compiler seems to implement char as equal to signed char .

• Because of the above, char should never be used for anything else but strings. It is bad practice to use for arithmetic operations. Such code relies on impl.defined behavior.

• The type int is always equal to signed int . This is required by the C standard.

• Thus on your specific system, the code you have written is equal to: (signed int)(signed char)200 .

• When you attempt to store the value 200 in a variable equivalent to signed char , it will overflow and get interpreted as -56 (on a two's complement system).

• When you cast a signed char containing -56 to a signed int , you get -56.

Answer 3

The number 200 is out of the value range for signed char . It wraps around into the negative value range in a two's complement way.

Obviously the (implicit) signedness of char is defined to be signed on your platform. Note that this is not mandated by the standard. If it were unsigned , your program would print 200. For further reading on this topic:

• Can someone explain how the signedness of char is platform specific?

Obviously, the size of the char type is 8 bits on your platform. The definition of this quantity is unspecified by the language specification as well. If it were larger, the program may very well print 200, just as you expected. For further reading on this topic:

• Is char guaranteed to be exactly 8-bit long?

Also note, that it is not specified by the standard that integer types must use the two's complement binary representation. On a different platform, the cast may even produce a completely different result. For further reading:

• http://en.wikipedia.org/wiki/Signed_number_representations
• http://en.wikipedia.org/wiki/Signed_number_representations#Comparison_table

Answer 4

char is signed , and int is signed (by default on your platform).

(signed char)11001000 = -56 decimal

(signed int)0000000011001000 = 200 decimal

Have a look at Signed number representations

Answer 5

char是-128〜127的8位有符号，所以200超出了值范围